RAID levels

RAID 0 - Striping

In a RAID 0 system data are split up into blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, this offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.

Advantages of RAID 0

  • RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.

  • All storage capacity is used, there is no overhead.

  • The technology is easy to implement.

Disadvantages of RAID 0

  • RAID 0 is not fault-tolerant. If one drive fails, all data in the RAID 0 array are lost. It should not be used for mission-critical systems.

Summary

  • Fastest Read/Write

  • Lowest reliability in case of failure, if 1 disk fails, all data is lost, not suitable for production


Ideal Use

RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, such as on an image retouching or video editing station.

If you want to use RAID 0 purely to combine the storage capacity of twee drives in a single volume, consider mounting one drive in the folder path of the other drive. This is supported in Linux, OS X as well as Windows and has the advantage that a single drive failure has no impact on the data of the second disk or SSD drive.


RAID 1 - Mirror

Data are stored twice by writing them to both the data drive (or set of data drives) and a mirror drive (or set of drives). If a drive fails, the controller uses either the data drive or the mirror drive for data recovery and continuous operation. You need at least 2 drives for a RAID 1 array.

Ideal for 2 disk setup only, for more than 2 disks use RAID 6 or RAID 10

Advantages of RAID 1

  • RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single drive.

  • In case a drive fails, data do not have to be rebuild, they just have to be copied to the replacement drive.

  • RAID 1 is a very simple technology.

Disadvantages of RAID 1

  • The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data get written twice.

  • Software RAID 1 solutions do not always allow a hot swap of a failed drive. That means the failed drive can only be replaced after powering down the computer it is attached to. For servers that are used simultaneously by many people, this may not be acceptable. Such systems typically use hardware controllers that do support hot swapping.

Summary

  • Low latency and high write speed

  • max fault tolerance of 1 disk

  • can only use half of potential storage

Ideal use

RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two data drives will be used.

RAID 5 Striping with Parity (Deprecated - dont use)

RAID 5 is the most common secure RAID level. It requires at least 3 drives but can work with up to 16. Data blocks are striped across the drives and on one drive a parity checksum of all the block data is written. The parity data are not written to a fixed drive, they are spread across all drives, as the drawing below shows. Using the parity data, the computer can recalculate the data of one of the other data blocks, should those data no longer be available. That means a RAID 5 array can withstand a single drive failure without losing data or access to data. Although RAID 5 can be achieved in software, a hardware controller is recommended. Often extra cache memory is used on these controllers to improve the write performance.

Advantages of RAID 5

  • Read data transactions are very fast while write data transactions are somewhat slower (due to the parity that has to be calculated).

  • If a drive fails, you still have access to all data, even while the failed drive is being replaced and the storage controller rebuilds the data on the new drive.

Disadvantages of RAID 5

  • Drive failures have an effect on throughput, although this is still acceptable.

  • This is complex technology. If one of the disks in an array using 4TB disks fails and is replaced, restoring the data (the rebuild time) may take a day or longer, depending on the load on the array and the speed of the controller. If another disk goes bad during that time, data are lost forever.

Ideal use

RAID 5 is a good all-round system that combines efficient storage with excellent security and decent performance. It is ideal for file and application servers that have a limited number of data drives.


RAID 6

RAID 6 is like RAID 5, but the parity data are written to two drives. That means it requires at least 4 drives and can withstand 2 drives dying simultaneously. The chances that two drives break down at exactly the same moment are of course very small. However, if a drive in a RAID 5 systems dies and is replaced by a new drive, it takes hours or even more than a day to rebuild the swapped drive. If another drive dies during that time, you still lose all of your data. With RAID 6, the RAID array will even survive that second failure.

Advantages of RAID 6

  • Like with RAID 5, read data transactions are very fast.

  • If two drives fail, you still have access to all data, even while the failed drives are being replaced. So RAID 6 is more secure than RAID 5.

Disadvantages of RAID 6

  • Write data transactions are slower than RAID 5 due to the additional parity data that have to be calculated. In one report I read the write performance was 20% lower.

  • Drive failures have an effect on throughput, although this is still acceptable.

  • This is complex technology. Rebuilding an array in which one drive failed can take a long time.


Summary

  • fast read, very slow write

  • fault tolerant, can tolerate 2 disk failure in 4 disk array

  • good for long term storage (NAS) where there is no requirement for high speed and high throughput writes


Ideal use

RAID 6 is a good all-round system that combines efficient storage with excellent security and decent performance. It is preferable over RAID 5 in file and application servers that use many large drives for data storage.



RAID 10

It is possible to combine the advantages (and disadvantages) of RAID 0 and RAID 1 in one single system. This is a nested or hybrid RAID configuration. It provides security by mirroring all data on secondary drives while using striping across each set of drives to speed up data transfers.

Advantages of RAID 10

  • If something goes wrong with one of the disks in a RAID 10 configuration, the rebuild time is very fast since all that is needed is copying all the data from the surviving mirror to a new drive. This can take as little as 30 minutes for drives of 1 TB.

Disadvantages of RAID 10

  • Half of the storage capacity goes to mirroring, so compared to large RAID 5 or RAID 6 arrays, this is an expensive way to have redundancy.


Summary

  • High write speed

  • Very expensive solution, uses only half of total disk space

  • Fault tolerant, 4 disk array can fail 2 drives w/o data loss

  • on a 4 disk array, RAID10 is better option than RAID6